Switching equipment

ABSTRACT

Switching equipment with a contactor (CE) and a circuit breaker (BR) located ahead of the contactor. For detection of welded-together contacts of the contactor, this is provided with means (SC) adapted, a certain time after an opening order to the contactor, to apply a voltage pulse to the operating coil (12) of the contactor and to compare the current response of the operating coil with a comparison level for forming a detection signal (s d ), which is supplied to the circuit breaker. Upon detection of welded-together contacts, the detection signal triggers an opening of the circuit breaker for disconnection of the contactor.

TECHNICAL FIELD

The invention relates to switching equipment with an electromagneticcontactor and a circuit breaker which is located ahead of the contactor.The contactor has an operating magnetic circuit with a magnetic core, anoperating coil and an armature which moves in dependence on the currentflow through the operating coil. Furthermore, the contactor has a numberof contacts which are influenced by the armature.

BACKGROUND OF THE INVENTION

Electromagnetic contactors are known and have been used for a long time,for example as switching means between a voltage source and an electricmotor. One problem with such contactors is that one or a few of thecontact pairs of a contactor may become fixed to each other by welding,and the risk of this is greater at high currents. Such welding togetherof contact pairs may, for example, be caused by contact bouncing whenclosing the contactor towards a high making current of an electricmotor.

The fact that one or more contact pairs become fixed by welding mayentail serious harmful effects. Upon an opening signal to a contactorwith a welded-together contact pair, the armature will move a certaindistance in the opening direction, because of the resilience in themechanical coupling, and then stop in an intermediate position. This maycause arcs in the contact pairs which are not welded together, and fire,explosion or other damage to the contactor and other equipment. In manyapplications, it may also, and independently thereof, cause seriousconsequences that a contactor does not open when, according to asupplied opening signal, it should have opened.

SUMMARY OF THE INVENTION

The object of the invention is to provide switching equipment of thekind mentioned in the introductory part of the description, in which therisk of damage and other inconvenience, which may otherwise arise duringan incomplete opening of the contactor caused by welded-togethercontacts, is eliminated in a simple manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail in the following withreference to the accompanying FIGS. 1-4 wherin.

FIG. 1 shows switching equipment according to the invention, connectedin the supply conduit of an ac motor.

FIG. 2 shows the composition of the control equipment of the contactor.

FIG. 3 shows the control circuit included in the control equipment.

FIG. 4 shows how some of the quantities occurring in the switchingequipment vary with time during an opening operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows switching equipment according to the invention connected tothe line between a three-phase motor M and an alternating-voltage powersupply network N. The switching equipment comprises contactor equipmentCE and a circuit breaker BR located ahead of the contact equipment (by"ahead of" is meant that the circuit breaker is arranged between thecontactor equipment and the supply network.) The function of theswitching equipment is to connect, in dependence on a control signals_(c), the motor to or disconnect the motor from the supply voltage. Thecontrol signal may be obtained in a known manner from superordinatecontrol equipment or be supplied manually. The contactor equipment isusually adapted to also to serve as thermal overload protection meansfor the motor and then receives an opening signal from a current-sensingprotective circuit (not shown). The circuit breaker BR, which in a knownway is adapted to trip at overcurrents, serves as overcurrent protectiondevice. As shown in the figure, the circuit breaker also receives atripping signal s_(d) from the contactor equipment for opening of thecircuit breaker if contacts of the contactor have become fixed bywelding.

In the usual manner, the contactor equipment has a bank of contacts 10which, in the three-phase application shown, has three contacts, one foreach phase. Via a resilient mechanical link 14, the contacts aremechanically connected to the armature 13 of the operating magnet 11 ofthe contactor, which magnet has an operating coil 12. The contactorequipment has control equipment SC which receives the control signals_(c). Upon signals for closing, the control equipment feeds a current Ito the operating coil and maintains this current at a desired value.Further, the control circuit comprises circuits for detecting contactswhich have become fixed by welding and for supplying a detection signals_(d) for tripping the circuit breaker BR if it is detected thatcontacts have become fixed by welding.

FIG. 2 shows the composition of the control equipment SC. The operatingcoil 12 is connected, in series with a resistor R1, a switchingtransistor TR1 and a measuring resistor R_(m), to a supply voltagesource with a direct voltage +U. A bypass diode D is connected inparallel with the operating coil. A measuring voltage u_(m),corresponding to the current I through the coil (in case of anon-conducting diode D), is obtained across the measuring resistor. Thetransistor TR1 is used, in the manner which will be described below, tocontrol the current through the coil 12 upon closing of the contactorand in the closed position, as well as for applying a voltage pulse tothe coil for detection of contacts being fixed by welding. An RC circuitcomprising a resistor R_(C) and a capacitor C is connected to the supplyvoltage source. The capacitor may be connected to the measuring resistorwith the aid of a switching transistor TR2. A control circuit CCreceives the control signal s_(c) and the measurement signals u_(m) andu_(c), the latter corresponding to the capacitor voltage, and deliverscontrol signals s_(I) and s_(rs) to the transistors TR1 and TR2 and thetripping signal s_(d) to the circuit breaker BR.

FIG. 3 shows the composition of the control circuit CC. The measurementsignal u_(m) is supplied to an input of a level-sensing circuit NV1, andto the second, inverting input there is supplied a reference value I₀which corresponds to the desired current through the operating coil 12when the contactor is closed. The circuit NV1 has a certain hysteresisand delivers an output signal which becomes "0" if the coil currentrises above an upper limit value and which becomes "1" if the currentdrops below a lower limit. The output signal of the circuit is forwardedvia an AND circuit OG1 to an OR circuit EG, the output signal s_(I) ofwhich controls the transistor TR1, which is on at s_(I) =1 and off ifs_(I) =0. The AND circuit releases the signal from NV1 and hence thecontrol signals to the transistor if there is an order for a closedcontactor, that is, if the control signal s_(c) is "1". The circuitdescribed so far thus controls, in a known manner, by pulsing thetransistor TR1, the current through the operating coil to the desiredvalue independently of supply voltages varying within wide limits.Circuits of this kind for control of the current through the operatingcoil of a contactor are known, for example from the published patentapplications EP 0 136 968 A3 and WO 86/01332.

The control signal s_(c) is also supplied to a monostable circuit MV1which is triggered when the control signal changes from "1" to "0", thatis, when an opening signal is supplied to the contactor. The circuit MV1then delivers a pulse with a duration t₁ so adjusted that the contactorhas normally had time to assume the open position at the end of thepulse. The output signal from the circuit MV1 is supplied to twoadditional monostable circuits MV2 and MV3, which are both triggered atthe end of the pulse from MV1, that is, the time t₁ after an openingorder to the contactor. The circuit MV2 delivers a short control pulses_(rs) to the transistor TR2, which thereby becomes conducting for ashort moment and causes the capacitor voltage u_(c) to become identicalwith the voltage u_(m) across the measuring resistor. The circuit MV3delivers a pulse with the duration t₂ which corresponds to the length ofthe detection interval and which, for example, may be 0.1 ms. This pulseis supplied to the transistor TR1 via the OR circuit EG and controls thetransistor to a conducting state for the duration of the pulse. In thisway, the supply voltage U is continuously applied to the operating coil12 for the duration of the detection pulse. The pulse from the circuitMV3 is also supplied to a fourth monostable circuit MV4, which istriggered at the end of the pulse from MV3, that is, at the end of thedetection interval, and then delivers a short signal to a second ANDcircuit OG2.

A level-sensing circuit NV2 is supplied with the signals u_(c) andu_(m), the latter with reversed sign. If u_(c) >u_(m), the output signalof the circuit is "1", and when, at the end of the detection interval,the circuit OG2 receives a pulse from the circuit MV4, a signal s_(d) isdelivered which indicates whether any of the contacts of the contactorhas been fixed by welding. This signal is supplied to the circuitbreaker BR and triggers an immediate opening of the circuit breaker.

FIG. 4 illustrates the process of some of the quantities occurring inthe switching equipment. At the top in the figure, the control signals_(c) is shown, which is "1" up to t=t₀, that is, for t≦t₀ the contactoris in the closed position. The control equipment controls the current Ithrough the operating coil by pulsing the transistor TR1, the controlsignal s_(I) of which is shown below the control signal s_(c) in thefigure. Below this, the current I is shown and as is clear from thediagram this is controlled so that its mean value corresponds to thereference value I₀.

At t=t₀ an opening order is given, and the control signal s_(c) becomes"0". The coil current I then decreases exponentially towards zero.

After the time t₁ determined by the circuit MV1, the detection intervalis started. A short control pulse s_(rs) is supplied to the transistorTR2, which becomes conducting and causes the capacitor voltage u_(c) tobecome identical with the measuring voltage u_(m). At the same time, thetransistor TR1 is controlled to the conducting state and the supplyvoltage U is applied to the operating coil. Its current I then increasesat a rate which is dependent on the magnitude of the supply voltage andon the inductance of the operating coil (the coil resistance is assumedto be constant). The inductance, in its turn, is dependent on thereluctance (the magnetic resistance) of the magnetic circuit of theoperating magnet. The reluctance varies, in turn, with the air gapbetween the armature and the magnetic core. It is smallest in a fullyclosed position, when the air gap is zero, and greatest in a fullyopened position when the air gap has its greatest value. If one or moreof the contacts of the contactor should be fixed by welding upon anopening operation, the armature, because of the resilient mechanicalcoupling between the armature and the contacts, will move a certaindistance until the welded contact or contacts prevent continuedmovement. The armature then stops in an intermediate position, where thereluctance assumes a value between its greatest and its smallest value.

The two lowermost diagrams in FIG. 4 show how the current I and themeasurement signal u_(m) vary during the detection interval. The normalprocess is shown in dotted lines. The air gap has had time to assume itsgreatest value even at the beginning of the detection interval, thereluctance is great and the coil inductance small, and therefore thecoil current increases rapidly. The unbroken lines show the process ifat least one contact is fixed by welding. The reluctance then becomeslower and the coil inductance greater, and the current increases moreslowly. The time constant of the RC circuit RC-C is so chosen that thesignal u_(c) increases more slowly than the coil current in the normalcase but faster than the coil current in case of a contact which isfixed by welding. At the end of the detection interval, therefore, inthe normal case u_(m) >u_(c) and no output signal is obtained from thecircuit NV2. In the case of a welded contact, on the other hand, at theend of the interval u_(m) <u_(c), the output signal from the circuit NV2becomes "1" and a tripping signal s_(d) is delivered to the circuitbreaker BR. This causes the circuit breaker to immediately trip andprevent further damage to the contactor and damage to the otherequipment.

By supplying the RC circuit in the above-described embodiment from thesame supply voltage source as the operating coil, the importantadvantage is obtained that variations in the supply voltage willinfluence the rate of growth of the comparison quantity u_(c) in thesame way and to the same extent as the variations influence the rate ofgrowth of the coil current. The detection of contacts fixed by weldingtherefore becomes correct even if the supply voltage varies, andswitching equipment according to the invention may be connected todifferent supply voltages without influencing the detection.

By setting the comparison quantity u_(c), at the beginning of thedetection interval, always equal to the value which corresponds to thecoil current, the detection becomes correct independently of themagnitude of the coil current at the beginning of the interval. This isan important advantage and makes it possible, for example, withoutnegatively influencing the accuracy of the detection, to initiate thedetection, and when necessary achieve disconnection of the contactor,earlier than what would otherwise have been possible, thus reducing theharmful effects of contacts fixed by welding.

From experience, in a typical contactor, the reluctance in the openposition is about 3-10 times greater than in the closed position, thatis, the coil inductance is about 3-10 times lower. This relatively largeratio makes possible a reliable detection of contacts fixed by weldingby utilizing a reluctance determination. Further, the method describedabove is simple and economically advantageous. It requires notransducers or extra connections of the contactor and only a relativelysimple supplementation of the static parts of the contactor equipment.In the case described above, where the invention is applied to contactorequipment which is provided with means for control of the current of theoperating coil, the already existing control means are utilized, and theonly thing that is required is a moderate supplementation of thesignal-processing circuits of the equipment.

The equipment described above is only an example, and switchingequipment according to the invention can be designed in a plurality ofother ways than that described above.

According to the invention, the change in the reluctance of theoperating magnet, in dependence on the position of the armature, isutilized for the detection. Quantities equivalent to the reluctance may,of course, alternatively be used within the scope of the invention, forexample the inverted value of the reluctance, the permeance, or the coilinductance proportional to the permeance.

In the above description, the operating coil and its current-controllingmeans have been used for the reluctance determination, which is a simpleand advantageous embodiment, but alternatively there may be used, forexample, a separate inductance measuring coil.

In the embodiment described above, a measure of the reluctance is formedby determining the current change during a time interval of apredetermined length. Alternatively, of course, a measure of thereluctance may be formed by determining the time for a predeterminedcurrent change.

The resetting of the comparison quantity (by closing the transistor TR2)described above causes the measurement to be completely independent ofwhich value the current coil has at the beginning of the detectioninterval.

The invention has been described above with reference to a contactor,the contacts of which are open when the contactor is in the openposition and closed in the closed position. The invention can also beapplied to a contactor with at least some contact which is closed in theopen position of the contactor and where thus the contactor, when thiscontact has been fixed by welding, may stop in an intermediate positionwhen closing the contactor.

In the embodiment described above, the control and detection equipmentis a mixture of analog and digital circuits, but the correspondingfunctions may be obtained in other ways, for example with the aid of anappropriately programmed microprocessor.

We claim:
 1. Switching equipment with an electromagnetic contactor and acircuit breaker located ahead of the contactor, the contactor having anoperating magnetic circuit with a magnetic core, an operating coil, anarmature which moves in dependence on the current through the operatingcoil, and a number of contacts which are influenced by the armature, theswitching equipment further comprising detection circuits for sensingthe reluctance of the operating magnetic circuit and, in dependence onthe measured reluctance, generating a signal which indicates anincomplete opening of the contactor caused by welded-together contacts,said signal being supplied to the circuit breaker so that, upondetection of welded-together contacts, the contactor is disconnected byopening of the circuit breaker, wherein the detection circuits measurethe reluctance of the operating magnetic circuit by sensing theinductance of an inductance measuring coil surrounding the magneticcore, and wherein the detection circuits apply to the inductancemeasuring coil a voltage pulse and detect the inductance of the coil onthe basis of the current response of the coil.
 2. Switching equipmentaccording to claim 1, wherein the inductance measuring coil consists ofthe operating coil.
 3. Switching equipment according to claim 1, whereinthe detection circuits sense the reluctance of the operating magneticcircuit when a time interval has elapsed after an opening order receivedby the contactor.
 4. Switching equipment according to claim 1, whereinthe detection circuits, at a predetermined time after the start of thevoltage pulse, compare the current response with a reference level. 5.Switching equipment according to claim 4, wherein the detection circuitsapply said voltage pulse to the inductance measuring coil by connectingthe coil to a voltage source and the detection circuits comprise meansfor forming the reference level in dependence on the voltage of thevoltage source.
 6. Switching equipment according to claim 1, wherein thedetection circuits, when reaching a predetermined current level, comparethe time interval elapsed since the start of the voltage pulse with apredetermined time interval.
 7. Switching equipment according to claim1, wherein the operating coil is connected to a voltage source in serieswith a switching member for controlling the current through the coil,and the detection circuits further comprise means adapted forcontrolling the switching means into a conducting state for applyingsaid voltage pulse across the operating coil.